A substrate transporter must be highly efficient while producing minimal contamination to the cleanroom environment. With regard to efficiency, both the throughput and the repeatability must be taken into consideration. For a substrate transporter, throughput may be determined by the swap time, and the time it takes the transporter to move a substrate between a container, e.g., a front opening universal pod (FOUP), a pre-aligner, and a chuck. Additionally, minimizing the release of particulates produced by the transporter into an equipment front end module (EFEM) is an important concern in many applications.
Recently, substrate transporters have been made from selective compliant articulated robot arms (SCARAs). The SCARA design provides a sealed enclosure for the moving portions of the robot in order to minimize the potential for contamination. However, while SCARAs do reduce the release of particulates, there are also drawbacks to the use of SCARAs. First, SCARAs are slow at moving substrates. The slow speed is partially due to the need to perform complex calculations in order to properly position the end effector. In order to complete a task, a SCARA must first compute a motion plan. Thereafter, the SCARA must use inverse kinematics to transform the motion plan into joint actuator trajectories for the robot.
Additionally, even when a SCARA is equipped with two end effectors, the swap times are still slow. FIGS. 1A-D are overhead views of a double end effector robot swapping a pair of substrates 110A and 110B at a chuck 115 according to the prior art. End effectors 112A and 112B are configured such that they are attached to a single arm of a substrate transporter. FIG. 1A depicts the upper end effector 112A retrieving a substrate 110A from a chuck 115. The lower end effector 112B is positioned below and oriented 90° away from the upper end effector 112A. This positioning is required in order to allow the upper end effector to freely access the first substrate 110A. The arrow indicates the direction of motion of both of the end effectors 112A and 112B. At FIG. 1B, the upper end effector 112A has picked up the first substrate 110A and the robot arm begins to remove the substrate 110A by retracting as indicated by the arrow. Once the upper end effector 112A has sufficient clearance, the end effectors 112A and 112B reverse their orientation to each other. FIG. 1C depicts the switched orientation that now allows the lower end effector 112B to have clear access to the chuck 115. Once reoriented, both end effectors are moved toward the chuck as indicated by the arrow. Finally, at FIG. 1D, the second substrate 110B has been positioned on the chuck 115, and the robot arm may retract the end effectors 112A and 112B so they are both clear from the chuck as shown by the arrow. As shown, this process contains wasted movement. The robot arm must insert and retract the end effectors 112A and 112B twice in order to swap the pair of substrates 110A and 110B.
SCARAs also have limited repeatability. SCARAs typically are limited to a repeatability of around 200 μm. It is widely known in the industry that the vibrational motion in SCARAs is a major limiting factor to the repeatability. This problem has increased in importance as the size of substrates has grown. Larger substrates, such as 300 mm and 450 mm substrates, result in longer arms for the substrate transporters and therefore more vibrations. As substrates continue to increase in size, problems with repeatability will continue to grow as well. Designs directed at reducing the vibrational motion often rely on increasing the stiffness of the structural materials. However stiffer materials are more expensive and increase the overall cost of production. Additionally, stiffening the structure by adding mass will result in more vibrations.
Therefore, there is a need in the art for a highly repeatable substrate transporter, with a reduced swap time. Further there is a need in the art for a substrate transporter that is highly efficient while still minimizing the release of particulates into the environment.